FIG. 11 shows a prior art microwave amplifier circuit. In the figure, reference numeral 1 designates an input terminal, numeral 2 designates a microwave amplifier, and numeral 3 designates an output terminal.
A description is given of the operation. An input signal applied to the input terminal 1 is amplified by the microwave amplifier 2, and the amplified output signal is output to the output terminal 3. Such a microwave amplifier circuit has characteristics such that when the level of the input signal increases, the amplified output signal becomes saturated at a prescribed level.
FIG. 12(a) shows a circuit of a dividing and combining type amplifier including two such microwave amplifier circuits and FIG. 12(b) shows results of simulation of the input and output levels of this circuit. In figure 12(a), reference numerals 1, 2, and 3 designate the same elements as in FIG. 11. Reference numeral 4 designates a divider which divides the input signal into two and numeral 6 designates a combiner for combining the output signals from the two microwave amplifiers 2.
A description is given of the operation. An input signal applied to the input terminal 1 is divided by the divider 4 into two, these two signals are amplified by the two microwave amplifiers 2, respectively, and the two amplified output signals are combined by the combiner 6 and its output is output from the output terminal 3.
In this dividing and combining type amplifier, when the input level increases exceeding some value, for example, about -22.0 dBm, and the output signal begins to be saturated, i.e., the output Pout becomes almost 0.0 dBm, the output levels of the doublet component 2f.sub.0 and the triplet component 3f.sub.0 become high relative to the fundamental wave component f.sub.0. When the output levels of the doublet component 2f.sub.0 and the triplet component 3f.sub.0 are high, the output distortion becomes large.
Such a microwave amplifier is used in a mobile communication system such as a portable telephone. In accordance with a rapid spread of recent mobile communication systems, such a microwave amplifier has been required to provide an output having low distortion.
In the prior art microwave amplifier described above, however, when the output signal is saturated because of an increase in the output level, the output level of the harmonic wave component increases, thereby resulting in large distortion in the output.
In addition, when the doublet component and the triplet component increase with the saturation of the output signal as described above, there occur phase variations which are caused by the passage of the signal through the amplifier.
Furthermore, when there are a lot of harmonic wave components in the amplified signal and the frequency signals having center frequencies of f1 and f2 quite close to each other are mixed, an output called third intermediate wave (hereinafter, referred to as IM3) is generated and has a frequency represented by 2.times.f1-f2 or 2.times.f2-f1.